Perpendicular magnetic recording system is adopted as a technique for increasing the magnetic recording density. A perpendicular magnetic recording medium at least comprises a non-magnetic substrate, and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer formed of a soft magnetic material and playing a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; an interlayer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
Japanese Patent Laid-Open No. 2001-291230 (PTL1) discloses a granular magnetic material as a material for forming the magnetic recording layer of the perpendicular magnetic recording medium. The granular material comprises magnetic crystal grains and a non-magnetic body segregated to surround the magnetic crystal grains. Magnetic crystal grains in the granular magnetic material are magnetically separated from each other by the non-magnetic body.
For the purpose of further increasing the recording density of the perpendicular magnetic recording medium, an urgent need for reduction in the grain diameter of the magnetic crystal grains in the granular magnetic material arises in recent years. On the other hand, reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (signals). Thus, the magnetic crystal grains in the granular magnetic material need to be formed of materials with higher magnetocrystalline anisotropies, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains.
One of proposed materials having the required higher magnetocrystalline anisotropies is L10 type ordered alloys. Japanese Patent Laid-Open No. 2004-178753 (PTL2) discloses L10 type ordered alloys comprising at least one element selected from the group consisting of Fe, Co, and Ni and at least one element selected from the group consisting Pt, Pd, Au and Ir, and a method for producing the alloys. Typical L10 type ordered alloys include FePt, CoPt, FePd, CoPd, and the like.
In order to obtain the ordered alloys, not only a film-forming process at an elevated temperature, but also a special interlayer which grows the ordered structure are necessary. Against this problem, International Patent Publication No. WO 2011/132747 (PTL3) discloses a magnetic recording layer comprising an FePt alloy having the L10 type ordered structure and a metal oxide such as ZnO, and a method for manufacturing the magnetic recording layer. The problem to be solved by this proposal is to form the L10 type FePt thin film onto a polycrystalline surface such as amorphous thermal silicon oxide (SiO2), for example, at a temperature as low as possible, in which a special crystal face and/or a surface treatment on the substrate made of metal or glass is unnecessary. In this method, the metal which constitutes the metal oxide is selected based on a melting point and an oxide formation free energy. Thereby, it is made possible to facilitate migration of the metal atoms in the FePt alloy even at a low temperature, thereby forming the L10 type ordered structure by rapid heating treatment for a short time.
On the other hand, reduction in the sizes of the magnetic crystal grains means reduction in the cross-sectional areas of the crystal magnetic grains having a certain height, since the thickness of the magnetic recording layer is basically uniform in in-plane directions of the medium. Therefore, a diamagnetic field acting on the magnetic crystal grains themselves decreases, whereas a magnetic field required for switching the magnetization of the magnetic crystal grains (magnetic switching field) increases. As described above, the improvement of the recording density implies that a larger magnetic field is required for recording signals, in view of the shape of the magnetic crystal grains.
Energy-assisted magnetic recording systems such as a heat-assisted recording system or a microwave-assisted recording system have been proposed as the other means against the problem of increase in the magnetic field strength required for recording (see NPL1). The heat-assisted recording system utilizes the temperature dependence of the magnetic anisotropy constant (Ku) of a magnetic material, which is a characteristic where the higher the temperature, the lower the Ku. This system uses a head having functions to heat a magnetic recording layer. That is, this system executes writing while reducing a magnetic switching field by raising the temperature of the magnetic recording layer to temporarily reduce the Ku. The recorded signals (magnetization) can be maintained stably, since the Ku returns its original high value after the temperature of the magnetic recording layer drops. In the application of the heat-assisted system, a magnetic recording layer needs to be designed taking its temperature characteristics into consideration, in addition to the conventional design guidelines.
The granular structure makes it possible to reduce magnetic interaction among the magnetic crystal grains for reducing a magnetization transition noise or the like, thereby improving a signal-to-noise ratio (SNR). On the other hand, uniform formation of the magnetic crystal grains is required, due to stringent demands on variation in magnetic properties among the respective magnetic crystal grains. However, when the magnetic crystal grains are formed of the ordered alloy, there is involved a difficulty in forming magnetic crystal grains with uniform properties. In order to alleviate the demands on uniformity of the magnetic crystal grains, a method of forming a magnetically continuous layer onto the magnetic layer having the granular structure is proposed. Japanese Patent Laid-Open No. 2013-168197 (PTL4) discloses a magnetic recording medium comprising a magnetic recording layer of a two-layered structure which consists of a first magnetic layer of a granular structure and a second magnetic layer of an amorphous structure. The purpose of this proposal is to reduce dispersion in a magnetic switching field by providing moderate magnetic interaction among the magnetic crystal grains in the first magnetic layer by the second magnetic layer. As one of constitutional examples, there is a description about an example in which the first magnetic layer has a stacked structure of a lower magnetic layer and an upper magnetic layer. The lower magnetic layer comprises an L10 type FePt alloy as a main component, and contains C. The upper magnetic layer comprises the L10 type FePt alloy as a main component and contains at least one component consisting of SiO2, TiO2, ZnO, and the like. The second magnetic layer comprises Co as a main component, and contains 6 to 16% by atom of Zr, and at least selected from the group consisting of B and Ta. It is explained that the first magnetic layer having the two-layered structure is effective in reducing dispersion in particle diameter and improving the SNR, and the amorphous second magnetic layer is effective in reducing dispersion in magnetic switching field (SFD), in the above constitution.
It is known that a difficulty is involved in forming a magnetic film having an ordered structure such as L10 type in high quality, and irregular growth of crystals is likely to occur during the film formation. This is because the non-magnetic material for separating the magnetic crystal grains not only fills the gaps between the magnetic crystal grains but also covers the top surface of the magnetic crystal grains. Japanese Patent Laid-Open No. 2011-154746 (PTL5) proposes a method for alleviating the irregular growth of the crystals by gradually decreasing the material for forming the grain boundary among the magnetic crystal grains along with the progress of the film formation. The magnetic recording medium comprises a substrate, a first magnetic layer of a granular structure which comprises magnetic crystal grains consisting of an L10 type ordered alloy, and a grain boundary-segregating material, and a second magnetic layer of an amorphous material. Here, the content of the grain boundary-segregating material in the first magnetic layer decreases continuously or stepwise, from the substrate to the second magnetic layer. It is described that control of the content of the grain boundary-segregating material affords a columnar structure due to continuous growth in a perpendicular direction, even when reducing a grain diameter of the magnetic crystal grains.